Drumlins Research Articles

Controls on coastal bluff erosion of a drowned drumlin field: Boston Harbor, Massachusetts

Eroding coastal bluffs pose significant risks to coastal communities, and an improved understanding of their erosional processes and mechanisms is essential for developing effective management strategies. The Boston Harbor Islands comprise more than two dozen drowned drumlins that were formed during the late-Pleistocene, many of which are now connected by spits. Exposed bluffs on the islands experience erosion due to waves and precipitation and are increasingly threatened by accelerating sea-level rise (SLR). Retreat rates for 31 bluffs along the drumlin shore, derived from orthoimagery (1995–2021) using the Digital Shoreline Analysis System add-in for ArcMap 10.8, range from 0.0 m/yr to 1.5 m/yr. These rates were examined in relation to several factors, including shoreline orientation, significant wave height during extratropical storms, till matrix composition, base elevation, bluff height, and presence of engineering structures. Clusters of bluffs with similar characteristics were identified, suggesting that high retreat rates coincide with bluffs that experience high wave energy, face the northeast and have high sand-to-mud ratios. The complexity of the Boston Harbor drumlin field and the variability of individual factors controlling erosion highlights the impracticality of developing a universal statistical model for predicting erosion rates. Rather a more suitable approach involves assessing trends among clusters of bluffs with similar characteristics. A similar approach can be taken for exploring the variability in retreat rates along other coastlines with complex or irregular bluff systems and wave fields.

Absent drumlins beneath southern lobes of the Laurentide Ice Sheet: A new hypothesis based on Des Moines Lobe dynamics inferred from landforms

AbstractSpatial distributions of drumlin fields encode information about ice sheet dynamics. No drumlins formed beneath the most lobate parts of the Laurentide Ice Sheet's southern margin, in South Dakota, Iowa and Illinois, whereas ice lobes to the northeast generally produced drumlins. This pattern may have resulted from northerly ice overriding permafrost. Here we propose a new hypothesis for this pattern by constructing a LiDAR‐based landform map and applying a model of drumlin formation to account for the absence of drumlins beneath the largest of the ice sheet's southern lobes, the Des Moines Lobe. Broad belts of hummocky topography, ice‐walled lake plains, doughnuts and minor moraines, which together cover 90% of the lobe's upland area in Iowa, attest to widespread ice stagnation, as does the lobe's scarcity of eskers. Most stagnation topography is subtle, with insufficient relief to have obscured drumlins that might have formed before stagnation. Minor moraines are crevasse‐squeeze ridges diagnostic of surging, and their ubiquity indicates that during surging, the lobe's soft bed was weak nearly everywhere. End moraines are generally parallel to a minor moraine setup‐glacier, implying that surge‐driven advances were more numerous than indicated by the three major end moraines of the lobe. In the only physically based model of drumlin formation that includes surging, till deposition occurs during surges when effective pressure is uniformly low, whereas drumlins develop during quiescent flow between surges, when basal slip and low‐pressure R‐channels create the spatial gradients in effective pressure necessary to sculpt drumlins by differential erosion. Landforms of the lobe, however, indicate stagnation and down‐wasting during quiescence, without significant basal slip or hydraulic potential gradients necessary for R‐channels. We hypothesize that for the three southernmost lobes of the Laurentide Ice Sheet, surging followed by widespread down‐wasting of stagnant ice prevented drumlin formation, whereas beneath northern lobes it was permitted by climatically‐forced ice advance.

Numerical modelling of subglacial ribs, drumlins, herringbones, and mega‐scale glacial lineations reveals their developmental trajectories and transitions

AbstractInitially a matter of intellectual curiosity, but now important for understanding ice‐sheet dynamics, the formation of subglacial bedforms has been a subject of scientific enquiry for over a century. Here, we use a numerical model of the coupled flow of ice, water, and subglacial sediment to explore the formation of subglacial ribs (i.e., ribbed moraine), drumlins and mega‐scale glacial lineations (MSGLs). The model produces instabilities at the ice–bed interface, which result in landforms resembling subglacial ribs and drumlins. We find that a behavioural trajectory is present. Initially subglacial ribs form, which can either develop into fields of organized drumlins, or herringbone‐type structures misaligned with ice flow. We present potential examples of these misaligned bedforms in deglaciated landscapes, the presence of which means caution should be taken when interpreting cross‐cutting bedforms to reconstruct ice flow directions. Under unvarying ice flow parameters, MSGLs failed to appear in our experiments. However, drumlin fields can elongate into MSGLs in our model if low ice–bed coupling conditions are imposed. The conditions under which drumlins elongate into MSGLs are analogous to those found beneath contemporary ice streams, providing the first mechanism, rather than just an association, for linking MSGLs with ice stream flow. We conclude that the instability theory, as realized in this numerical model, is sufficient to explain the fundamental mechanics and process‐interactions that lead to the initiation of subglacial bedforms, the development of the distinctive types of bedform patterns, and their evolutionary trajectories. We therefore suggest that the first part of the longstanding ‘drumlin problem’ – how and why they come into existence – is now solved. However, much remains to be discovered regarding the exact sedimentary and hydrological processes involved.

Drumlin formation within the Bustarfell drumlin field, northeast Iceland: integrating sedimentological and ground‐penetrating radar data

ABSTRACTDrumlins are important bedforms of former glaciated landscapes as they demonstrate past ice‐flow directions and elucidate processes that operated at the ice/bed interface. Recently mapped drumlins and other streamlined subglacial bedforms in northeast Iceland reveal the flow‐sets of cross‐cutting palaeo‐ice streams that were active within the Iceland Ice Sheet (IIS) during and following the Last Glacial Maximum. Here we study the Bustarfell drumlin field within the Vopnafjörður–Jökuldalsheiði flow‐set. The internal architecture of two drumlins was investigated using sedimentological analysis and ground‐penetrating radar (GPR, 50 and 100 MHz) to illuminate subglacial processes that contributed to drumlin formation, as well as the history and dynamics of the IIS. On the stoss side of one of the drumlins, two subglacial traction till units were identified, separated by a thick unit of deformed glaciofluvial sand and gravel. The core of glaciofluvial material suggests that the drumlin formed around well‐drained patches (sticky spots) in the subglacial bed that retarded the ice flow locally through increased basal drag and encouraged till deposition. Furthermore, our GPR data indicate a combination of erosional and depositional processes. We suggest that the glaciofluvial sediments were deposited as small ice‐marginal fans on the Bustarfell plateau, possibly during the Bølling–Allerød interstadial, and that the drumlins were formed around these fans during a subsequent readvance during the Younger Dryas.

Origin of glacial curvilineations by subglacial meltwater erosion: Evidence from the Stargard drumlin field, Poland

AbstractDeciphering the origin of subglacial landforms is crucial for understanding processes operating under ice sheets. Due to the inaccessibility of the ice/bed interface limiting real‐time observations, the origin of these landforms is typically interpreted from the morphological and sedimentological record exposed after ice retreat, which makes the conclusions conjectural. Here, we focus on a recently discovered new type of subglacial landform called glacial curvilineations (GCLs), whose formation generates some controversy. We use LiDAR imagery and geological data to constrain the origin of GCLs found in a tunnel valley in the Stargard drumlin field, Poland. The tunnel valley is about 3800 m long, 520 m wide, and it occupies about 2 km2. A total of 61 individual GCLs have been mapped there; they are up to 550 m long and typically 0.5–1.5 m high, but some reach a height of up to 5 m. The GCLs are composed of mostly massive till represented by either a single or several diamicton units occasionally separated by thin layers of meltwater sand. Till macrofabrics in the GCLs and in the nearby till plain are distinctly consistent, and have high and uniform clustering strengths. At all locations, the dominant till fabric direction broadly corresponds with the ice movement direction given by the drumlin orientations, but it is oblique to the orientation of the tunnel valley and independent of the orientations of the individual GCLs. The lack of correspondence between the orientation of the GCLs and the till fabrics in them suggests that the GCLs consist of an antecedent till deposited during the drumlinizing ice advance and subsequently carved by subglacial meltwater erosion in the tunnel valley. These results substantiate the hypothesis that GCLs are erosional features postdating the material constituting them, and emphasize the role of subglacial meltwater flows as a landforming agent.

Till kinematics in the Stargard drumlin field, NW Poland constrained by microstructural proxies

ABSTRACTDeciphering the relationships between streamlined subglacial landforms and deposits therein helps to constrain the dynamics of past ice sheets. Here we present microstructural analyses of till from the largest drumlin field in the North European Lowland generated by a major palaeo‐ice stream of the Scandinavian Ice Sheet. Data derived from thin sections and X‐ray microtomography (μCT) reveal multiple microstructures including microshears, circular structures, grain stacks and crushed grains. Long axes of micro‐clasts measured in thin sections are directionally clustered and have low, down‐ice‐oriented dip angles. Some samples reveal subordinate clustering oriented at high angles to the main direction. The μCT data show low clustering strengths of clasts and no systematic fabric pattern in the entire set of samples. There is no correspondence between the clast orientations and the directions of drumlin long axes, suggesting that the till is antecedent in relation to the drumlin‐shaping process. The observations indicate spatially and temporally variable, shallow‐seated brittle and ductile deformation controlled by porewater pressure fluctuations and generated time‐transgressively parallel to till accretion. Consistently with previous studies, the data suggest that the Stargard drumlins are erosional remnants of a pre‐existing terrain shaped by material removal from the inter‐drumlin areas.

Insights into drumlin development from ground-penetrating radar at Múlajökull, Iceland, a surge-type glacier

AbstractDrumlins form at the ice/bed interface through subglacial processes that are not directly observable. The internal stratigraphy of drumlins provides insight into how they developed and associated subglacial processes, but traditional stratigraphic logging techniques are limited to natural exposures and excavations. Using ground-penetrating radar, we imaged the internal stratigraphy of seven drumlins from a recently exposed drumlin field in the forefield of Múlajökull, Iceland. Data were collected with 100 and 200 MHz antennas with maximum resolvable depths of 8 and 4 m, respectively. Longitudinal echograms contained coherent down-ice dipping reflectors over the lengths of the drumlins. Near the drumlin heads (i.e., stoss sides), down-glacier dipping beds lie at high angles to the surface, whereas on the lee sides, the down-glacier dipping beds lie at low angles, or conform, to drumlin surfaces. Transverse echograms exhibited unconformities along the flanks of drumlin heads and conformable bedding across the lee side widths of the drumlins. These observations were ground-truthed with stratigraphic logs from a subset of drumlins and good agreement was found. The stratigraphic patterns support previous conclusions that drumlins at Múlajökull formed on a deformable bed through both depositional and erosional processes which may alternate between its surge and quiescent phases.

Groundwater Flow Under a Paleo‐Ice Stream of the Scandinavian Ice Sheet and Its Implications for the Formation of Stargard Drumlin Field, NW Poland

AbstractSubglacial groundwater flow was an integral part of glaciological systems of past ice sheets, but its impact on the origin of active‐ice landforms remains unexplored. Using numerical experiments, we attempt to constrain groundwater flow dynamics under a major paleo‐ice stream of the southern Scandinavian Ice Sheet and its impact on the formation of the Stargard drumlin field. Flow models show a total reorganization of groundwater dynamics under the advancing ice stream to a depth of up to ~200 m. A mosaic of intervening groundwater recharge and discharge areas originates, whereby the same areas may experience multiple shifts in flow directions. A prominent time‐ and space‐transgressive pressure pump recharges groundwater in a subglacial zone up to about 20 km within the ice margin and discharges it in front of the ice sheet. The simulated groundwater flow pattern suggests that drumlins occupying a portion of the ice stream area occur preferentially where groundwater upwells and discharges at the ice/bed interface. Consistent with the geological composition, a possible drumlin‐forming mechanism involves an excess of pressurized water under the ice reducing the strength of the subglacial deposits and facilitating glacial erosion, erosion by turbulent meltwater flows or both, streamlining antecedent deposits into drumlin shapes. This study emphasizes the importance of causal feedback between subglacial groundwater flow and the ice sheet dynamics and suggests an impact of the former on the formation of streamlined subglacial landforms in general.

An erosional origin for drumlins of NW Poland

AbstractDrumlins are landforms essential to understanding of ice sheet movement over soft beds, sediment transport along the ice/bed interface, and the formation of a wide range of glacial deposits. Although investigated more than any other glacial landform, the origin of drumlins remains contentious. Using high‐resolution LiDAR imagery and field data, we investigate the geomorphology and internal composition of one of the biggest drumlin fields in the North European Lowland. The Stargard drumlin field consists of over 1300 drumlins and related streamlined subglacial bedforms in a terminal part of a major Weichselian palaeo‐ice stream of the southern Scandinavian Ice Sheet. The drumlins are typically 600‐800 m long, 200‐250 m wide, 3‐6 m high and have axial elongation ratios ~2 but in some cases exceeding 15. Several subzones inferred from drumlin morphometry exist reflecting different ice flow dynamics. The most elongated drumlins occur in areas where ice moved down‐slope and where thick fine‐grained deposits of low hydraulic conductivity occur in the substratum. The largest portion of land occupied by drumlins and the greatest frequency density of drumlins occur where the ice moved up‐slope. Stargard drumlins are composed of a wide variety of glacial deposits including various types of tills and meltwater sediments, which range from undisturbed to heavily deformed. There is no correlation between the deposits in the drumlins and the drumlin forms indicating that the deposits pre‐date the drumlinizing process. It is suggested that the drumlin field was generated by a combination of direct glacial erosion and subglacial meltwater erosion by removing antecedent material from the inter‐drumlin areas and streamlining the resultant bumps. Our data support the search for a unifying theory of drumlin formation and suggest erosion as the most plausible single mechanism generating drumlin landscapes. © 2019 John Wiley & Sons, Ltd.

Subglacial and ice‐marginal landforms in south‐central Ontario: implications for ice‐sheet reconfiguration during deglaciation

Regional‐scale, high‐resolution terrain data permit the study of landforms across south‐central Ontario, where the bed of the former Laurentide Ice Sheet is well exposed and passes downflow from irregular topography on Precambrian Shield highlands to flat‐lying Palaeozoic carbonate bedrock, and thick (50 to >200 m) unconsolidated sediment substrates. Rock drumlins and megagrooves are eroded into bedrock and mega‐scale glacial lineations (MSGL) occur on patchy streamlined till residuals in the Algonquin Highlands. Downflow, MSGL pass into juxtaposed rock and drift drumlins on Palaeozoic bedrock and predominantly till‐cored drumlins in areas of thick drift. The Lake Simcoe Moraines, now traceable for more than 80 km across the Peterborough drumlin field (PDF), form a distinct morphological boundary: downflow of the moraine system, drumlins are larger, broader and show no indication of subsequent reworking by the ice, whereas upflow of the moraines, a higher degree of complexity in bedform pattern and morphology is distinguished. Discrete radial and/or cross‐cutting flowset terminate at subtle till‐cored moraine ridges downflow of local topographic lows, indicating multiple phases of late‐stage ice flow with strong local topographic steering. More regional‐scale flow switching is evident as NW‐orientated bedforms modify drumlins south of the Oak Ridges Moraine, and radial flowset emanate from areas within the St. Lawrence and Ottawa River valleys. Most of the drumlins in the PDF formed during an early, regional drumlinization phase of NE–SW flow that followed the deposition of a thick regional till sheet. These were subsequently modified by local‐scale, topographically controlled flows that terminate at till‐cored moraines, providing evidence that the superimposed bedforms record dynamic ice (re)advances throughout the deglaciation of south‐central Ontario. The patterns and relationships of glacial landform distribution and characteristics in south‐central Ontario hold significance for many modern and palaeo‐ice sheets, where similar downflow changes in bed topography and substrate lithology are observed.

Measuring Hyperscale Topographic Anisotropy as a Continuous Landscape Property

Several landforms are known to exhibit topographic anisotropy, defined as a directional inequality in elevation. The quantitative analysis of topographic anisotropy has largely focused on measurements taken from specific landforms, ignoring the surrounding landscape. Recent research has made progress in measuring topographic anisotropy as a distributed field in natural landscapes. However, current methods are computationally inefficient, as they require specialized hardware and computing environments, or have a limited selection of scales that undermines the feasibility and quality of multiscale analyses by introducing bias. By necessity, current methods operate with a limited set of scales, rather than the full distribution of possible landscapes. Therefore, we present a method for measuring topographic anisotropy in the landscape that has the computational efficiency required for hyperscale analysis by using the integral image filtering approach to compute oriented local topographic position (LTP) measurements, coupled with a root-mean-square deviation (RMSD) model that compares directional samples to an omnidirectional sample. Two tools were developed: One to output a scale signature for a single cell, and the other to output a raster containing the maximum anisotropy value across a range of scales. The performances of both algorithms were tested using two data sets containing repetitive, similarly sized and oriented anisotropic landforms, including a dune field and a drumlin field. The results demonstrated that the method presented has the robustness and sensitivity to identify complex hyperscale anisotropy such as nested features (e.g., a drumlin located within a valley).

Production and preservation of the smallest drumlins

Few very small drumlins are typically mapped in previously glaciated landscapes, which might be an important signature of subglacial processes or an observational artefact. One hundred and forty-three newly emergent drumlins, recently sculpted by the Múlajökull glacier, have been mapped using high-resolution LiDAR and aerial photographs in addition to field surveying. In this paper, these are used as evidence that few small drumlins (e.g., height H 4 m, width W 40 m, length L 100 m) are produced; at least, few survive to pass outside the ice margin in this actively forming drumlin field. Specifically, the lack of a multitude of small features seen in other landforms (e.g., volcanoes) is argued not to be due to (i) Digital Elevation Model resolution or quality, (ii) mapper ability in complex (i.e., anthropogenically cluttered or vegetated) landscapes or (iii) post-glacial degradation at this site. So, whilst detection ability must still be at least acknowledged in drumlin mapping, and ideally corrected for in quantitative analyses, this observation can now be firmly taken as a constraint upon drumlin formation models (i.e., statistical, conceptual or numerical ice flow). Our preferred explanation for the scarcity of small drumlins, at least at sites similar to Múlajökull (i.e., ice lobes with near-margin drumlin genesis), is that they form stochastically during multiple surge cycles, evolving from wide and gentle pre-existing undulations by increasing rapidly in amplitude before significant streamlining occurs.

A Theoretical Model of Drumlin Formation Based on Observations at Múlajökull, Iceland

AbstractThe drumlin field at the surge‐type glacier, Múlajökull, provides an unusual opportunity to build a model of drumlin formation based on field observations in a modern drumlin‐forming environment. These observations indicate that surges deposit till layers that drape the glacier forefield, conform to drumlin surfaces, and are deposited in shear. Observations also indicate that erosion helps create drumlin relief, effective stresses in subglacial till are highest between drumlins, and during quiescent flow, crevasses on the glacier surface overlie drumlins while subglacial channels occupy intervening swales. In the model, we consider gentle undulations on the bed bounded by subglacial channels at low water pressure. During quiescent flow, slip of temperate ice across these undulations and basal water flow toward bounding channels create an effective stress distribution that maximizes till entrainment in ice on the heads and flanks of drumlins. Crevasses amplify this effect but are not necessary for it. During surges, effective stresses are uniformly low, and the bed shears pervasively. Vigorous basal melting during surges releases debris from ice and deposits it on the bed, with deposition augmented by transport in the deforming bed. As surge cycles progress, drumlins migrate downglacier and grow at increasing rates, due to positive feedbacks that depend on drumlin height. Drumlin growth can be accompanied by either net aggradation or erosion of the bed, and drumlin heights and stratigraphy generally correspond with observations. This model highlights that drumlin growth can reflect instabilities other than those of bed shear instability models, which require heuristic till transport assumptions.

Spatial organization of drumlins

AbstractIce‐sheets flowing over soft sediments produce undulations in the bed, typically of metres in relief, of which drumlins are the most abundant and widely investigated. Consensus regarding their mechanism of formation has yet to be achieved. In this paper we examine the spatial organization of drumlins in order to provide an improved description of the phenomenon and to guide hypotheses of their formation. We review the literature highlighting contradictory findings regarding drumlin spatial organization and then use this to motivate our study based on a large sample (42 488) of drumlins from Canada, Britain and Norway. Are there typical arrangements in drumlin positioning and are they organized in a regular spatial manner (patterned) or are they distributed randomly? We recognize that drumlin fields are inherently patchy and therefore apply inhomogeneous spatial statistics in order to study their distribution. This shows that whilst drumlins are occasionally randomly placed, their main state is non‐ random. They exhibit a strong and statistically significant signal of regularity across lengths scales of 100 to 1200 m. We conclude that patterning is a near ubiquitous property of drumlins. This finding of regularity demonstrates spatial self‐organization in the bedforming process with drumlins as an emergent manifestation of sub‐glacial sediment mobility. Kilometre‐scale interactions between drumlins must occur as they evolve, or interactions may arise as a consequence of growth or migration. Hypotheses or models are required that can explain the regular spacing of drumlins. We highlight three suggestions for such self‐organization: instability in the coupling of ice flow–sediment flux–bed shape; local feedback between sediment mobility and relief; and coarsening by growth or migration. © 2017 The Authors. Earth Surface Processes and Landforms published by John Wiley & Sons Ltd.

Automated delineation and characterization of drumlins using a localized contour tree approach

Drumlins are ubiquitous landforms in previously glaciated regions, formed through a series of complex subglacial processes operating underneath the paleo-ice sheets. Accurate delineation and characterization of drumlins are essential for understanding the formation mechanism of drumlins as well as the flow behaviors and basal conditions of paleo-ice sheets. Automated mapping of drumlins is particularly important for examining the distribution patterns of drumlins across large spatial scales. This paper presents an automated vector-based approach to mapping drumlins from high-resolution light detection and ranging (LiDAR) data. The rationale is to extract a set of concentric contours by building localized contour trees and establishing topological relationships. This automated method can overcome the shortcomings of previously manual and automated methods for mapping drumlins, for instance, the azimuthal biases during the generation of shaded relief images. A case study was carried out over a portion of the New York Drumlin Field. Overall 1181 drumlins were identified from the LiDAR-derived DEM across the study region, which had been underestimated in previous literature. The delineation results were visually and statistically compared to the manual digitization results. The morphology of drumlins was characterized by quantifying the length, width, elongation ratio, height, area, and volume. Statistical and spatial analyses were conducted to examine the distribution pattern and spatial variability of drumlin size and form. The drumlins and the morphologic characteristics exhibit significant spatial clustering rather than randomly distributed patterns. The form of drumlins varies from ovoid to spindle shapes towards the downstream direction of paleo ice flows, along with the decrease in width, area, and volume. This observation is in line with previous studies, which may be explained by the variations in sediment thickness and/or the velocity increases of ice flows towards ice front.

Semi-automated extraction of longitudinal subglacial bedforms from digital terrain models – Two new methods

Relict drumlin and mega-scale glacial lineation (positive relief, longitudinal subglacial bedforms – LSBs) morphometry has been used as a proxy for paleo ice-sheet dynamics. LSB morphometric inventories have relied on manual mapping, which is slow and subjective and thus potentially difficult to reproduce. Automated methods are faster and reproducible, but previous methods for LSB semi-automated mapping have not been highly successful. Here, two new object-based methods for the semi-automated extraction of LSBs (footprints) from digital terrain models are compared in a test area in the Puget Lowland, Washington, USA. As segmentation procedures to create LSB-candidate objects, the normalized closed contour method relies on the contouring of a normalized local relief model addressing LSBs on slopes, and the landform elements mask method relies on the classification of landform elements derived from the digital terrain model. For identifying which LSB-candidate objects correspond to LSBs, both methods use the same LSB operational definition: a ruleset encapsulating expert knowledge, published morphometric data, and the morphometric range of LSBs in the study area. The normalized closed contour method was separately applied to four different local relief models, two computed in moving windows and two hydrology-based. Overall, the normalized closed contour method outperformed the landform elements mask method. The normalized closed contour method performed on a hydrological relief model from a multiple direction flow routing algorithm performed best. For an assessment of its transferability, the normalized closed contour method was evaluated on a second area, the Chautauqua drumlin field, Pennsylvania and New York, USA where it performed better than in the Puget Lowland. A broad comparison to previous methods suggests that the normalized relief closed contour method may be the most capable method to date, but more development is required.

My Mother, in Drumlin Country

My Mother, in Drumlin Country

The Generation of Mega Glacial Meltwater Floods and Their Geologic Impact

A mechanism is presented explaining how mega meltwater avalanches could be generated on the surface of a continental ice sheet. It is shown that during periods of excessive climatic warmth when the continental ice sheet surface was melting at an accelerated rate, self-amplifying, translating waves of glacial meltwater emerge as a distinct mechanism of meltwater transport. It is shown that such glacier waves would have been capable of attaining kinetic energies per kilometer of wave front equivalent to 12 million tons of TNT, to have achieved heights of 100 to 300 meters, and forward velocities as great as 900 km/hr. Glacier waves would not have been restricted to a particular locale, but could have been produced wherever continental ice sheets were present. Catastrophic floods produced by waves of such size and kinetic energy would be able to account for the character of the permafrost deposits found in Alaska and Siberia, flood features and numerous drumlin field formations seen in North America, and many of the lignite deposits found in Europe, Siberia, and North America. They also could account for how continental debris was transported thousands of kilometers into the mid North Atlantic to form Heinrich layers. It is proposed that such layers’ form at times when a North Atlantic sea ice shelf borders the ice sheet and when climate warms abruptly producing accelerated meltwater discharge.

Origin of the active drumlin field at Múlajökull, Iceland: New insights from till shear and consolidation patterns

Abstract Stratigraphic and morphologic data previously collected from the forefield of Mulajokull, Iceland, suggest that its recent surge cycles are responsible for the formation of drumlins there and that their relief reflects both deposition on drumlins and erosion between them. We have tested these ideas and aspects of leading models of drumlin formation by studying past patterns of bed deformation and effective stress in basal tills of the glacier's forefield. Patterns of till strain indicated by the anisotropy of magnetic susceptibility (AMS) of ∼2300 intact till samples indicate that till was deposited during shear deformation, with shearing azimuths and planes that conform to the drumlin morphology. Thus, till deposition occurred as drumlins grew, in agreement with LiDAR data indicating that the degree of aggradation of the glacier forefield is largest in areas subjected to the most surges. Previously described unconformities on the drumlin flanks, however, indicate that drumlin relief at Mulajokull has resulted, in part, from erosion. Given that the last surge deposited a till layer both on and between drumlins, a reasonable hypothesis is that erosion between drumlins occurred during normal (quiescent) flow of the glacier between surges. Densities of till samples, analyzed in conjunction with laboratory consolidation tests, indicate that effective stresses on the bed during such periods were on the order of 100 kPa larger between drumlins than within them, an observation consistent with subglacial channels at low water pressure occupying interdrumlin areas. Transport of sediment by turbulent flow in these channels or high effective stress adjacent to them causing enhanced till entrainment in ice or increased depths of bed deformation would promote the sediment flux divergence necessary to erode areas between drumlins. The observation that effective stresses were higher between drumlins than within them is the opposite of that presumed in leading models of drumlin formation. Moreover, the lack of AMS-fabric evidence of longitudinal compression in drumlin tills does not support some models of drumlin formation that invoke negative till-flux gradients in a deforming bed.

Progressive formation of modern drumlins at Múlajökull, Iceland: stratigraphical and morphological evidence

The drumlin field at Múlajökull, Iceland, is considered to be an active field in that partly and fully ice‐covered drumlins are being shaped by the current glacier regime. We test the hypothesis that the drumlins form by a combination of erosion and deposition during successive surge cycles. We mapped and measured 143 drumlins and studied their stratigraphy in four exposures. All exposures reveal several till units where the youngest till commonly truncates older tills on the drumlin flanks and proximal slope. Drumlins inside a 1992 moraine are relatively long and narrow whereas drumlins outside the moraine are wider and shorter. A conceptual model suggests that radial crevasses create spatial heterogeneity in normal stress on the bed so that deposition is favoured beneath crevasses and erosion in adjacent areas. Consequently, the crevasse pattern of the glacier controls the location of proto‐drumlins. A feedback mechanism leads to continued crevassing and increased sedimentation at the location of the proto‐drumlins. The drumlin relief and elongation ratio increases as the glacier erodes the sides and drapes a new till over the landform through successive surges. Our observations of this only known active drumlin field may have implications for the formation and morphological evolution of Pleistocene drumlin fields with similar composition, and our model may be tested on modern drumlins that may become exposed upon future ice retreat.